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  Experimental Study on the Effect of Localized Weak Zones on the Seismic Behavior of Shallow Tunnels

  Weak zones around the tunnel lining may be formed because of the poor workmanship, inadequate back-filling, weathering or erosion. The situation is more complicated as such weak zones formation behind the lining, may not be readily spotted until substantial damage to the lining has been produced. Several case histories showed that the main factors for the failure of tunnels were the weak zones behind the lining. This study aims to experimentally investigate the ground movement and the resulting stress redistribution in the liner due to the presence of deteriorated weak zones in shallow tunnels. The location of weak zones was considered as a variable along with the varying overburden depths under dynamic conditions. A 2-dimensional shaking table was used for the experimental setup. The results of the tests revealed that by placing the cavity at tunnel crown and springline a considerable ground movement was observed whose magnitude and direction depend on cavity location. Moreover, the bending moments in the liner were changed due to the presence of weak zones as compared to the case of full contact. Likewise, at some points the bending moment changed its direction, which can cause spalling, cracking of concrete and eventually the liner failure.

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